The invention relates to a sifting device for sifting granular material having a static cascade sifter as a pre-sifting chamber followed by at least one rod basket of a dynamic sifter member rotatably disposed in the sifter housing, provided with turbo elements on the rotor circumference, having inlets for sifting air/sifting gas and sifting material and outlets for sifting air/sifting gas, fine, medium and coarse material.
DE-A-42 23 762, which corresponds with U.S. Pat. No. 5,392,998 specifies a sifting device having a static sifter member with no moving parts disposed inside a common sifter housing and a dynamic sifter member with rotating rod basket. In this case, the static sifter member is formed as a pre-sifting chamber for separating the coarse grain fraction contained in the granular material to be sifted, having two sifting zone limiting walls enclosed by the shaft-shaped sifter housing and forming a sifting zone between them and flowed through by the sifting air in cross flow, where said walls have baffle plates sloping diagonally downward in the direction toward the discharge opening for the separated coarse grain fraction disposed cascade-like or louver-like, where both baffle plate walls, and thus the sifting zone in between, are disposed sloping at an angle deviating from vertical. Because the sifting air in the static sifting member is deviated in a V-shape this is also referred to as a so-called V-sifter. Such a static V-sifter, included in a circulation grinding system, for example, with a material comminution roller press or roller mill, was found to be effective for energy-saving sifting and deagglomerating the grinding material in one single work process.
If the grinding material is moist, which is the case most of the time with cement raw meal, for example, the grinding material can also be dried in the static V-sifter member, in addition to being sifted and deagglomerated, if hot gas is used as sifting air, such as the hot waste gas from a cement clinker production line. In this case in particular, in view of the drying, relatively large hot gas or sifting gas quantities have to be used which then require a respectively sized rod basket or several such rod baskets for the rod basket sifter in the dynamic sifter member connected directly to the static V-sifter member, which could impair the efficient operation of the dynamic sifter member that separates the sifting material, from which the coarse grain fraction has been removed, into a medium grain fraction and fine material.
Therefore, the aim of the invention is to create a sifting device of the above described type having a static sifter member directly followed by a dynamic sifter member with at least one rotatably disposed rod basket such that the two sifter members are optimally coordinated so as to operate efficiently, particularly with moist sifting material, namely the static V-sifter member as a pre-sifter and drier, if applicable, and the dynamic sifter member as an instrument for final sifting and final drying, if applicable.
The problem is solved in accordance with the invention by means of a sifting device.
The sifting device of the invention is characterized in that from the intermediate area between the static member and the dynamic member of the sifting device a bypass line leaves via which a partial flow of the sifting air flow or sifting gas flow exiting the static sifter member is drawn off, for example in a quantity of approx. 20 to 80% so that only a small amount of sifting air/sifting gas reaches the downstream dynamic sifter member with the rod basket, which, as a result, can be dimensioned smaller.
The solid material contained in the bypass flow, which will be a mixture of medium and fine material, is separated from the bypass flow in an external solid matter separator and can represent an additional product of the sifting device. It is of particular advantage, however, to resupply said product, which was separated from the bypass flow in the solid matter separator, via a solid matter discharge line to the dynamic sifter member so as to subject said resupplied product to a final sifting and a final drying, if applicable, in the dynamic sifter member.
According to a further characteristic feature of the invention, the bypass line can leave from the top side of the sifter housing, and the solid matter separator for the bypass line can then be disposed above the sifting device, while the solid matter discharge line of the solid matter separator is advantageously inserted into the dynamic sifter member from the top. However, another option is to insert the solid matter discharge of the external solid matter separator partially or completely into the static sifter member. Another option is to have the bypass line leave from a side wall of the sifter housing instead of from the top side and to dispose the solid matter separator for the bypass line below the dynamic sifter member, if appropriate.